The technology disclosed herein improves upon two existing technologies: (1) the steerable parasitic antenna, and (2) the phased array antenna. The state of the art for steerable parasitic antennas includes a cluster of antennas, where the main antenna is fed by an RF connection and the parasitic antennas are each fed by a tunable impedance device or variable phase element. In this prior art design, the coupling between the antenna elements is constant and is provided by free-space. The feed point impedance of each of the parasitic elements is tuned, and this changes the reflection coefficient of that element. In this way, the resulting beam can be steered.
The meta-element disclosed herein operates in a somewhat similar manner, but has several advantages. In the disclosed meta-elements, the feed point impedance of the parasitic elements is constant and the coupling coefficient is provided by a tunable device, rather than by free space. This provides three advantages:                (1) The coupling coefficient can be greater because of the presence of the tunable device, allowing the antenna to be lower profile than the prior art alternative. Free space coupling requires a minimum vertical length between adjacent elements to be exposed to each other, which sets the minimum height of these elements.        (2) The use of constant (rather than tunable) feed point impedance allows greater freedom in the design of the elements. In fact, elements with no RF feed point at all can be used. This allows greater simplicity and thus lower cost.        (3) This architecture provides additional degrees of freedom compared to the prior art architecture, which allows the meta-element to have greater capabilities in the forming and steering of beams and nulls.        
If M elements are arranged in a lattice, and each element has n neighbors, the prior art architecture only allows M degrees of freedom, because it is the feed-point impedance of each element that is tuned while the coupling is constant. With the architecture disclosed herein, there are potentially Men degrees of freedom because the coupling between each neighboring element can potentially be tuned separately. This greater freedom allows greater capabilities in controlling the beam angle(s), null angle(s), frequency response, and polarization of the antenna.
When used as an array of meta-elements, the disclosed meta-element provides an advantage over state-of-the-art phased arrays, because, among other things, it is simpler. It can be lighter and lower-cost, and can fill a greater number of applications. These improvements come about because the tunable coupling between the elements provides much of the beam steering and power distribution/collection of the array, thus reducing the number of required components such as phase shifters and power combiners or dividers. In addition, for the control system, a single analog line can take the place of several digital lines, reducing the total number of connections. For slow-speed scanning, the elements can be addressed by rows and columns, further simplifying the array.
The disclosed meta-element can be used in a number of applications, including next-generation vehicular communication systems, where beam steering may be needed for greater gain and for interference cancellation, low-gain steerable antennas on mobile platforms, or unmanned ground units. When used as an array of meta-elements, the technology disclosed herein can find a large number of applications as a replacement for conventional phased array antennas. Since it can be low profile and conformal, as well as low-cost, it can fit a wide variety of applications. Furthermore, there are many communication and sensing systems that are impractical today, but that would be enabled by the existence of a low-cost or lightweight phased array. For example, the ability to place a steerable, high-gain antenna on every vehicle on the battlefield would allow more sophisticated networks and enhanced data-gathering and coordination than is presently available. With a greater number of connected nodes, the value of a network is increased by the square of the number of nodes, as described by Metcalf's law.
The prior art includes existing parasitic antennas such as the Yagi-Uda array (see FIG. 1) and steerable versions such as the steerable parasitic array (see FIG. 2). It also includes phased arrays (see FIG. 9(a)). It also includes tunable impedance surfaces (see FIG. 4(a)—in the prior art the bias voltages are the same for all patches), which are one kind of a system of coupled radiators. It also includes traditional antennas consisting of systems of coupled oscillators (see FIG. 3), which are typically steered by pulling the phase of the edge elements, but often lacks a simple means of feeding the antenna with an arbitrary waveform or receiving a signal.
In general, steerable antennas are made up of several or many discrete antennas. Beam steering is typically accomplished by preceding each radiating antenna with a phase shifter. The phase shifters control the phase of the radiation from each antenna, and produce a wave front having a phase gradient, which results in the main beam being steered in a particular direction depending on the direction and magnitude of this phase gradient. If the spacing between the antennas is too large, a second beam will also be formed, which is called a grating lobe.
The minimum spacing to prevent grating lobes depends on the direction of the main beam, and it is between one-half wavelength and one wavelength. For large arrays, this results in a large number of antennas, each with its own phase shifter, resulting in a high cost and complexity. A feed structure is also required to feed all of these antennas, which further increases the cost and weight.
The prior art also includes a body of work that has appeared in various forms, and can be summarized as a lattice of small metallic particles that are linked together by switches. Such antennas can be considered as distinct from the present disclosure because the metal particles are not resonant structures by themselves, but only when assembled into a composite structure by the switches.
The prior art also includes:                1. B. Chiang, J. A. Proctor, G. K. Gothard, K. M. Gainey, J. T. Richardson, “Adaptive Antenna for Use in Wireless Communication Systems”, U.S. Pat. No. 6,515,635, issued Feb. 4, 2003;        2. M. Gabbay, “Narrowband Beamformer Using Nonlinear Oscillators”, U.S. Pat. No. 6,473,362, issued Oct. 29, 2002;        3. T. Ohira, K. Gyoda, “Array Antenna”, U.S. Pat. No. 6,407,719, issued Jun. 18, 2002;        4. R. A. Gilbert, J. L. Butler, “Metamorphic Parallel Plate Antenna”, U.S. Pat. No. 6,404,401, issued Jun. 11, 2002;        5. J. Rothwell, “Self-Structuring Antenna System with a Switchable Antenna Array and an Optimizing Controller”, U.S. Pat. No. 6,175,723, issued Jan. 16, 2001;        6. T. E. Koscica, B. J. Liban, “Azimuth Steerable Antenna”, U.S. Pat. No. 6,037,905, issued Mar. 14, 2000;        7. D. M. Pritchett, “Communication System and Methods Utilizing a Reactively Controlled Directive Array”, U.S. Pat. No. 5,767,807, issued Jun. 16, 1998;        8. J. Audren, P. Brault, “High Frequency Antenna with a Variable Directing Radiation Pattern”, U.S. Pat. No. 5,235,343, issued Aug. 10, 1993;        9. R. Milane, “Adaptive Array Antenna”, U.S. Pat. No. 4,700,197, issued Oct. 13, 1987;        10. L. Himmel, S. H. Dodington, E. G. Parker, “Electronically Controlled Antenna System”, U.S. Pat. No. 3,560,978, issued Feb. 2, 1971; and        11. Daniel Sievenpiper, U.S. Pat. No. 6,496,155.        